Electric relay



April 25, 1961 E. J. HOPKINS ETAL 2,981,867

ELECTRIC RELAY Filed Oct. 25, 1957 Inventors: Ernest J. Hopkins,

Ronald H. Magon, -M-Vf) b9 Thel A ttcJrT-reg.

2,981,867 ELECTRIC RELAY Ernest J. Hopkins, Drexel Hill, Pa., and Ronald H. Macphersou, Woodbury, NJ., assignors to General Electric Company, a corporation of New York Filed Oct. 25, 1957, Ser. No. 692,365

9 Claims. (Cl. 317-36) This invention relates to electric relays, and more particularly to a relay responsive to the direction of alternating current in a polyphase electric power transmission line.

A directional relay of conventional construction typically comprises an arrangement of electromagnets, a movable armature or an induction rotor, and contacts carried by the armature or rotor to perform a preselected control function whenever the relay operates. The electromagnets of the conventional relay are provided with windings connected to be energized by transmission line current and voltage, and these windings establish magnetic fluxes in proportion to their respective Venergizing quantities. The magnetic lluxes interact in the movable armature or rotor to produce a torque proportional to the vector product of the interacting uxes, the direction of torque being determined by the polarity of said product. Relay operation is obtained whenever the torque is in the proper direction to close the contacts. The operating characteristic of such a relay is conveniently expressed in terms of the phase relationship between line current and line voltage, i.e. the power factor angle, that will cause maximum contact closing torque. It should be apparent that the above described relay is sensitive to variations in the power factor angle and thus is capable of directional discrimination.

it is a general object of this invention to provide a relatively compact and inexpensive electric relay having the operating characteristic of a directional relay without utilizing the conventional structure described above.

It is another object of the invention to provide a general purpose relay without conventional electromagnets and movable armature or induction rotor.

Still another object of the invention is the provision of an improved directional relay having no moving parts.

In carrying out our invention Vin one form, a directional relay is adapted to control an electro-responsive device in response to predetermined voltage and current relationships in an alternating current electric power transmission line. The electroresponsive device is operable when energized to perform a preselected control function. In accordanceV with our invention, we provide means responsive to transmission line voltage and to a first alternating voltage having a predetermined constant relationship to transmission line current for producing a unipolarity operating voltage representative of the vector sum of said line voltage and the first alternating voltage. Also provided is means responsive to line voltage and to a second alternating voltage having a predetermined constant relationship to line current for producing a relatively smooth unipolarity reference voltage representative of the vector difference between the line voltage and the second alternating voltage. Coupling means interconnects both of the unipolarity voltage producing means and the electroresponsive device to form a loop circuit wherein the operating and reference voltages are oppositely poled, and the electroresponsive device is operably energized only when the magnitude of the opatenr O erating voltage exceeds the magnitude of the reference voltage.

Our invention will be better understood and its various objects and advantages will be more fully appreciated from the following description taken in conjunction with the accompanying drawing in which:

Fig. 1 is a circuit diagram illustrating a basic principle employed in our invention;

Fig. 2 is a schematic diagram of an electric relay enibodying the invention;

Fig. 3 is a vector diagram of certain voltage relation; ships obtained in the illustrated embodiment of the finvention;

Fig. 4 is a voltage-time diagram of certain voltage relationships in the illustrated embodiment; and n Fig. 5 is a voltage-phase angle diagram illustrating the operating characteristic of a directional relay constructed in accordance with our invention.

Referring now to the circuit diagram of Fig. l, the reference number 11 identifies the sensing element of an electroresponsive device to be operated. By energizing the sensing element 11 with unidirectional current I0 greater than a predetermined magnitude, operation of the electroresponsive device is obtained. Since it forms no part of our invention, the electroresponsive device itself has not been shown, and this device may beof any suitable type preferably characterized by extreme sensitivity. Accordingly, it may be assumed that said predetermined magnitude of unidirectional current is very low, eg. .O01 ampere. The internal impedance of the sensing element 11 is designated Ro, and this element is connected to a pair of terminals 12a and 12b. K

As can be seen in Fig. l, the terminals 12a and 12b and the sensing element 11 are connected in a unilaterally conductive loop circuit comprising a pair of input terminals 13a and 13b, a half-wave rectifier 14 and a load resistor 15. A unipolarity operating voltage V0 is applied across input terminals 13a and 13b, and by means of another pair of input terminals 16a and 16h, a unipolarity reference voltage Vr is applied across the load resistor 1S. The operating and reference voltages are poled so that input terminal 13a is positive with respect to terminal 13b and input terminal 16a is positive with respect to terminal 16b. In other words, the operating and reference voltages are applied to the terminals 12a and 12b in voltage opposing relationship, and thus the relative magnitudes of these two voltages determine the energization of the sensing element 11. The impedance of load resistor 15 is designated RL. It may be assumed that the forward impedance of rectier 14 as well as the impedanees of all remaining portions of the illustrated circuit are negligible.

Inspection of Fig. l reveals that current I0 flows only when the operating voltage Vo is greater than the reference voltage Vr, and that the magnitude of I0 is equal to VonVr Ro Since a very small magnitude of current I0 will operably energize the sensing element 11, operation of the electroresponsive device is obtained whenever V0 exceeds Vr by only a slight amount. Thus, the threshold of operation is reached when the operating and reference voltages are inst equal, and this critical relationship, as will be` come apparent hereinafter, is useful in representing the operating characteristic of the relay described below.

ln Fig. 2 we have shown an electric relay employing the basic principle discussed above. As shown, the relay is associated with a 3-wire polyphase alternating current electric power transmission line which is representeddby the three wires or phase conductors L1, L2 and L3. The relay is designed yto'operate in response to predetermined Ytransforming means 20.

`device may be used to perform a suitable control function, such as supervising` the operation of other protective relays not shown.

As is shown in Fig. 2, a current transformer 18 is coupled to wire L3 of the transmission line. Wire L3 comprises a single-turn primary winding for current trans- -former 18, and the secondary winding 19 of this transformer is connected to a primary winding 20a of suitable In some applications of our invention, it may be desired to energize the primary winding 20a in laccordance with the difference current 'owing in transmission line wires L1 and L2, and in such case a pair of current transformers suitably coupled to wires L1 and L2 could be provided in lieu of the arrangement shown.

For the purposes of the illustrated embodiment of our invention, the transforming means 20 preferably comprises the primary winding 20a, a pair of secondary windings 20b .and 20c and a common iron core 20d having at least one air gap. A potentiometer 21 having a slider 21a is connected across secondary winding 20b of the transforming means 20, and a suitable rheostat 22 having a slider 22a is connected across the secondary winding 20c.

Transforming means 20 derives across its secondary windings Ztlb and 20c alternating voltages representative, both in magnitude and phase, of the phase current I in the transmission line conductor L3 over the operating -range of current. In other words, the derived alternating voltages are related to Iby predetermined constant impedances. The angle of the constant vectorial relationship, i.e. the angle by which the derived alternating voltage leads the phase current, is determined by the total amount of load connected to the secondary windings, and by means ofthe rheostat 22 this angle may be varied as desired. Open circuit secondary voltage will lead phase current by 90 electrical degrees, and as the secondary load is increased the angle of lead becomes less. The transforming means 20 imposes minimum burden on current transformer 1S, and due to the high percentage of total primary current used for magnetizing iron core 20d and its air gap, no appreciable initial transient D.C. component of fault current will be reproduced in the alternating volta ge.

The peak magnitude of the alternating voltage across secondary winding 20c is all, where a1 is a predetermined constant in units of ohms. The magnitude of a, may be determined by appropriate selection of the turns ratio of the current transformer 18 and of transforming means 20. The derived voltage all leads phase current by a predetermined angle a which, as pointed out above, may be varied by appropriate adjustment of rheostat 22.

The peak magnitude of alternating voltage across the tapped Vportion of potentiometer 21 is al, where a is a predetermined constant in units of ohms. The magnitude of a may be equal to or different than al, according to the position of slider 21a. The derived voltage lz leads phase current by the same predetermined angle As can be s een in Fig. 2, the primary winding 23a of a potential transformer 23 is connected across wires L1 and L2 of the transmission line. Thus, the well known 90 degrees or quadrature coupling is used in connecting the primary windings of current and potential transformers 18 and 23 tothe transmission line. The secondary winding 23b of potential transformer 23 is connected to primary winding 24a of suitable auxiliary means or transformer 24. Auxiliary transformer 24 is provided with a pair of secondary windings 2412 and 24C, and the alternating voltage derived across each one of these secondary windings is a preselected portion of the phase-to-phase transmission line voltage E between wires L1 and L2. A potentiometer 25 having a slider 25a is connected across secondary winding 24b.

The peak magnitude of the alternating voltage across secondary winding 24e is CIE, where c1 is a fixed constant determined by the turns ratio of the potential transformer 23 and of auxiliary transformer 24, the latter preferably being 1:1. The peak magnitude of the voltage across the tapped portion of potentiometer 25 is cE, where c is a fixed constant which may be equal to or different than c1 as determined by the position of the slider 25a. Both of the alternating voltages CE and 61E are in phase with E.

Potentiometers 21 and 25 are connected in series circuit, voltage additive relationship. The respective circuits in Fig. 2 are poled so that the voltage which appears between slider 21a and slider 25a of potentiometers 21 and 25, respectively, comprises the vector sum of the derived voltages CEY and aI, or in other words cE-i-al. Another potentiometer 26 having a slider 26a is connected across the sliders 21a and 25a.

The tapped portion of potentiometer 26 is connected to the input circuit of suitable rectifying means, such as the illustrated full-wave bridge type rectifier 27. The peak magnitude of the voltage applied to the alternating current terminals of rectifiers 27 is equal to MCE-Htl', where b is a predetermined proportionality constant having a magnitude of unity or less as controlled by the setting of the slider 26a. The rectified voltage produced at the direct current terminals of rectifier 27, which voltage is an accurate representation of the vector sum of the transmission line voltage E and an alternating voltage having a predetermined constant relationship to transmission line current I, comprises the operating voltage of the illustrated relay. As will be apparent later in this specification, the operating voltage could also be obtained by connecting potentiometer 26 across thedirect currentA terminals of rectifier 27. With this alternative arrangement, it would be necessary to add a half-wave rectifier or the like in the circuit connected to slider 26a.

The secondary windings 20c and 24e` of transforming means 20 and auxiliary transformer 24, respectively, are connected in series circuit, voltage subtractive relationship. These series connected secondary windings are connected to the input circuit of suitable rectifying means, such as the full-wave bridge type rectifier 28 that is shown in Fig. 2. The respective circuits involved are poled so that the voltage which is applied to the alternating current terminals of rectifier 28 comprises the vector difference betweenl the derived voltages c1171 and alI, or in other words clE-all.

A suitable smoothing circuit is connected to the direct current terminals or output circuit of rectifier 28. The smoothing circuit may comprise, for example, a lowpass symmetrical T filter comprising a pair of series connected choke coils 29 and 30, an intermediate parallel connected capacitor 31, and a load resistor 32 connected in the manner shown in Fig. 2. The various components of the illustrated smoothing circuit may be selected to produce across load resistor 32 a direct voltage having a relatively small ripple factor, e.g. less than ten per cent, without adversely affecting the response time of the relay.

The magnitude of the relatively smooth direct voltage continuously developed across load resistor 32 is equal to d|c1E-a1l|, where d is a proportionality constant determined Yby the degree of liltering employed. The magnitude of d is less than but approaches unity. This direct voltage, which is an accurate representation of the vector difference between the transmission line voltage E and an alternating voltage having a predetermined constant relationship to transmission line current I, comprises the reference voltage of the illustrated relay. For the purposes of the present description, the

reference voltage may be assumed ideally smooth with negligible ripple factor.

The negative terminals of the rectitiers 27 and 28 and of load resistor 32 are all connected in common, as can be seen in Fig. 2. The positive direct current terminal of rectiier 27 is coupled to an output terminal 33a, and the positive terminal of load resistor 32 is coupled to another output terminal 33b. A sensing element 34 of the sensitive electromagnetic switching device 17 is connected between output terminals 33a and 33b. In accordance with the basic principle illustrated in Fig. 1 and discussed hereinbefore, the sensing element 34 will be operably energized when the magnitude of operating voltage exceeds the magnitude of reference voltage by a slight amount, and the threshold of operation obtains just as these two quantities reach their critical relationship of equality. This condition of equality is blc+al=dlclaa11t For a clearer understanding of our invention, the various voltage relationships described above have been illustrated graphically in Figs. 3 and 4. In the Fig. 3 vector diagram, the triangle l-2-3 represents the three phase-to-phase voltages of the transmission line. The vector represents the phase current flowing in transmission line wire L3 in the direction indicated by arrow 35 in Fig. 2, and this current lags its unity power factor position by a phase angle 0. The alternating voltages 1I and ff, which are derived by transforming means 26, lead current by the angle rp. The vector dilference between the derived voltage 21T and the preselected portion cl of the phase-tophase transmission line voltage between Wires L1 and L2, or in other words the difference voltage cl--l-I, is clearly indicated in magnitude ,dlcl-all of the reference voltage is slightly less than the peak magnitude cl-E--el of the difference voltage. The operating voltage Vo has been shown in Fig. 4 by plotting the instantaneous values of the rotating vector c| reduced by the proportionality constant b which is determined by potentiometer 26. For the particular conditions illustrated, the peak magnitude blc-f-Zl-f of the operating voltage is less than the magnitude of reference voltage.

In the unilaterally conductive loop circuit comprising sensing element 34, load resistor 32, rectier 27 and the tapped portion of potentiometer 26 of Fig. 2, the difference between the instantaneous operating voltage magnitude and the reference voltage, as long as the latter is greater, appears across the individual rectifying elements of the full-wave rectifier 27. Accordingly, the rectifier 2,7 is rendered non-conductive, output terminals 33a and 33b are at the same potential, and no current can viiow to energize the sensing element 34. But when operating voltage exceeds reference voltage, output terminal 33a of the illustrated relay will be positive with respect to output terminal 3311 during at least some portion of each half cycle, and the resulting intermittent voltage drop across terminals 33a and 33h, which voltage comprises the output control signal of our relay, causes corresponding pulses of current to flow in the sensing .element 34 of the switching device 17. The switching By equating these two quantities and reducing, the condition of equal operating and reference voltages of the Fig. 2 embodiment of our invention may be expressed as follows:

where Z is the ratio E/I. Equation 1 represents the operating characteristic of a general purpose impedance responsive distance relay. By an appropriate selection of constants, the above described relay may be designed to provide directional discrimination. By selecting La a1 C1 b Equation l may be written KEI sin (o-)=o (2) which characterizes a directional relay. When the phase angle 0 is equal to qs or qb-I-ISO", the peak magnitude of operating voltage will be just equal to the reference voltage magnitude. The peak magnitude of operating voltage will become greater than reference voltage, and the relay will produce an output control signal, whenever sin (t9-gb) is positive. The operating voltage is greatest with respect to reference voltage when 0=+90 (the angle of maximum torque). Thus, the output control signal of the relay is a function of the power factor or phase angle of the transmission line current.

The operating characteristic of the above described directional relay is illustrated in Fig. 5 where Vo- Vr is plotted against phase angle 0. The shaded area in Fig. 5 represents operating conditions. The angle (p has been selected to be nearly degrees, and zero phase angle corresponds to the flow of current I in the direction indicated by the arrow 35 in Fig. 2 under unity power factor conditions. It can be seen that a degree reversal in the phase position of I will result in relay operation.

While we have shown and described preferred forms of our invention by way of illustration, many modifications will occur to those skilled in the art. We therefore contemplate by the claims which conclude this specication to cover all such modications as fall within the true spirit and scope of our invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

l. In a relay for an alternating current electric power transmission line: rst and second output terminals; a lrst full-wave rectifier having positive and negative direct current terminals and a pair of alternating current terminals, said positive direct current terminal being connected to said first output terminal; transforming means responsive to transmission line current for deriving rst and second alternating voltages related to the line current by first and second predetermined constant impedances, respectively; iirst means coupled to the transmission line and to said transforming means for applying to the alternating current terminals of said rst rectifier a voltage proportional to the vector sum of a first preselected portion of transmission line voltage and said first derived voltage; a second full-wave rectifier having a pair of direct current terminals and a pair of alternating current terminals; second means coupled to the transmission line and to said transforming means for applying `to the alternating current terminals of said second rectiiier a voltage proportional to the vector difference between a second preselected portion of transmission line voltage and said second derived voltage; and a smoothing circuit connected to the direct current terminals of said second rectifier, said smoothing circuit including a load resistor across which is developed a relatively smooth direct voltage representative of the voltage applied to the alternating current terminals of said second rectifier, said load resistor having a negative terminal connected to said negative direct current terminal of said first rectifier and having a positive terminal connected to Asaid second output terminal; whereby an output control signal is developed at said output terminals whenever the magnitude of said direct voltage is less than the instantaneous magnitude of the voltage applied to the alternating current terminals of said first rectifier.

2. In a directional relay for a polyphase three-wire electric power transmission line: rst and second output terminals; a first full-wave rectifier having positive and negative direct current terminals and a pair of alternating current terminals, said positive direct current terminal being connected to said first output terminal; transforming means responsive to phase current in a first wire of the transmission line for deriving rst and second alternating voltages related to said phase current by a predetermined impedance; first means coupled to said transforming means and to the second and third wires of the transmission line for applying to the alternating current terminals of said first rectifier a voltage proportional to the vector sum of said first derived voltage and a preselected portion of phase-to-phase transmission line voltage, the peak magnitude of the voltage applied to the alternating current terminals of said first rectifier being related by a predetermined proportionality constant to the peak magnitude of the vector sum voltage; a second full-wave rectifier having a pair of direct current terminals and a pair of alternating current terminals; second means coupled to said transforming means and to the second and third wires of the transmission line for applying to the alternating current terminals of said second rectifier a voltage proportional to the vector difference between said second derived voltage and said preselected portion of phase-to-phase transmission line voltage; and a smoothing circuit connected to the direct current terminals of said second rectifier, said smoothing circuit -including a load resistor across which is developed a relatively smooth direct voltage, the magnitude of said direct voltage being related by said predetermined proportionality constant to the peak magnitude of the vector difference voltage, said load resistor having a negative terminal connected to said negative direct current terminal of said first rectifier and having a positive terminal connected to said second output terminal; whereby an output control signal is developed across said output terminals whenever the magnitude of said direct voltage is less than the instan. taneous magnitude of the voltage applied to the alternat- .ing current terminals of said first rectifier.

3. In a directional relay for a polyphase three-wire electric power transmission line: first and second output terminals; a first full-wave rectifier having positive and negative direct current terminals and a pair of alternating current terminals, said positive direct current terminal being connected to said first output terminal; transforming means responsive to phase current in a first wire of the transmission line for deriving first and second alternating voltages related to said phase current by first and second predetermined constant impedances, respectively; first means coupled to said transforming means and to the second and third wires of the transmission line for applying to the alternating current terminals of said first rectifier a voltage equal to the vector sum of said first derived voltage and a first preselected portion of phase-to-phase transmission line voltage; a second full-wave rectifier having a pair of direct current terminals and a pair of altercurrent terminals of said second rectifier a voltage proportional to the vector difference between said second derived voltage and a second preselected portion of phaseto-phase transmission line voltage, the ratio of said first preselected portion to said second preselected portion being equal to the ratio of said first predetermined impedance to said second predetermined impedance; and a smoothing circuit connected to the direct current terminals of said second rectifier, said smoothing circuit including a load resistor across which is developed a relatively smooth direct voltage, the magnitude of said direct Voltage being related by a predetermined proportionality constant to said vector difference, said preselected proportionality constant being equal to the ratio of said first predetermined impedance to said second predetermined impedance, said load resistor having a negative terminal connected to said negative direct current terminal of said first rectifier and having a positive terminal connected to said second output terminal; whereby an output control signal is developed at said output terminals whenever the magnitude of said direct voltage is less than the instantaneous magnitude of said vector sum.

4. In a relay for an alternating current electric power transmission line: a pair of rectifying means having alternating current and direct current terminals; first means coupled to the transmission line and to the alternating current terminals of a first one of said rectifying means for energizing said first rectifying means in accordance with the vector sum of transmission line voltage and a Afirst derived voltage having a predetermined constant repeak magnitude of the voltage at the alternating current terminals of said one rectifying means; a pair of output terminals; and circuit means, comprising said smoothing circuit and the direct current terminals of the other Vrectifying means interconnected in series and oppositely poled, connected to said output terminals for energizing said output terminals in accordance with the voltage difference between said direct voltage and the instantaneous magnitude of the voltage at the alternating current terminals of said other rectifying means whenever said voltage difference has a predetermined polarity.

5. In a relay adapted to energize an electro-responsive device in response to predetermined current and voltage relationships in a three-phase electric power transmission line: a potential transformer having primary and secondary windings; a current transformer having primary and secondary windings; the primary windings of said transformers being coupled to the transmission line in quadrature; means connected to said current transformer secondary winding for deriving first and second alternating voltages representative of transmission line current; rst rectifying means supplied by said first derived alternating voltage in additive relationship with the voltage of said potential transformer secondary winding for producing a unipolarity operating voltage; second rectifying means supplied by said second derived alternating voltage in subtractive relationship with the voltage of said potential transformer secondary winding for producing a relatively smooth unipolarity reference voltage; and unilaterally conductive circuit means, including said first and second rectifying means interconnected in series and oppositely poled, adapted for connection to the electroresponsive device to form a loop circuit wherein energizing current can flow whenever the magnitude of said operating voltage exceeds Vthe magnitude of said reference voltage.

6. In a directional relay for a three-phase electric power transmission line: a potential transformer having primary and secondary windings; -a current transformer having primary and secondary windings; the primary windings of said transformers being coupled to the transmission line in quadrature; transforming means connected to said cur rent transformer secondary winding for deriving first and second alternating voltages related to transmission line current by first and second predetermined impedances, respectively, auxiliary means connected to said potential transformer secondary winding for providing third and fourth alternating voltages related to transmission line voltage by first and second predetermined proportionality constants, respectively, the ratio of said first and second predetermined proportionality constants being equal to the ratio of said first and second predetermined impedances; first and second rectifying means having alternating current and direct current terminals; first means coupled to said transforming means and to said auxiliary means for applying to the alternating current terminals of said first rectifying means a voltage representative of the vector difference between said fourth and second voltages; a smoothing circuit connected to the direct current terminals of said first rectifying means to develop a relatively smooth direct voltage related by a third predetermined proportionality constant to the peak magnitude of the vector difference voltage; second means coupled to said transforming means and to said auxiliary means for applying to the alternating current terminals of said second rectifying means a voltage related by a fourth predetermined proportionality constant to the vector sum of said third and first voltages, the ratio of said third and fourth predetermined proportionality constants being equal to the ratio of said first and second predetermined impedances; a pair of output terminals; and circuit means, comprising said smoothing circuit and the direct current terminals of said second rectifying means interconnected in series and oppositely poled, for applyingto said output terminals the voltage difference between said direct voltage and the instantaneous magnitude of the voltage applied to the alternating current terminals of said second rectifying means whenever said voltage difference has a predetermined polarity.

7. In a directional relay for a three-phase electric power transmission line: a potential transformer having primary and secondary windings; a current transformer having primary and secondary windings; the primary windings of said transformers being coupled to the transmission line in quadrature; transforming means connected to said current transformer secondary winding for deriving first and second alternating voltages related to transmission line current by first and second predetermined impedances, respectively; auxiliary means connected to said potential transformer secondary winding for providing third and fourth alternating voltages related to transmission line voltage by first and second predetermined proportionality constants, respectively, the ratio of said first and second predetermined proportionality constants being equal to the ratio of said first and second predetermined impedances; first means coupled to said transforming means and t said auxiliary means for energization in accordance with the vector difference between said fourth and second voltages to produce a relatively smooth unipolarity reference voltage whose magnitude is related by a third predetermined proportionality constant to the peak magnitude of said vector difference; second means coupled to said transforming means and to said auxiliary means for energization in accordance with the vector sum of said third and first voltages to produce a unipolarity operating voltage 10 whose peak magnitude is related by a fourth predetermined proportionality constant to said vector sum, the ratio of said third and fourth predetermined proportionality constants being equal to the ratio of said first and second predetermined impedances; a pair of output terminals; and means connected to saidrfirst and second means for applying said operating and reference voltages to said output terminals in voltage opposing relationship.

8. A directional relay adapted to energize an electro responsive device in accordance with the direction of cur rent flow in one wire of a three-wire polyphase alternating current electric power transmission line, comprising: transforming means responsive to the transmission line current in said one wire for deriving first and second alternating voltages, the peak magnitudes of said first and second alternating voltages being related to said current by a predetermined impedance; first means responsive to the vector sum of said first alternating voltage and a preselected portion of the voltage across the other two wires of the transmission line for producing a unipolarity operating voltage, the peak magnitude of said operating voltage being related by a predetermined proportionality constant to the peak magnitude of said vector sum; second means responsive to the vector difference between said second alternating voltage and said preselected portion of the voltage across said other two wires of the transmission line for producing a relatively smooth unipolarity reference voltage, the magnitude of said reference voltage being related by said predetermined proportionality constant to the peak magnitude of said vector difference; and circuit means, incl-uding both of said unipolarity voltage producing means, for applying said operating and reference voltages in voltage opposing relationship to the electroresponsive device; whereby unidirectional energizing current will fiow to the electroresponsive device in a direction determined by the relative magnitudes of said `operating and reference voltages.

9. A directional relay adapted to energize an electroresponsive device in accordance with the direction of current flow in one wire of a three-wire polyphase alternating current electric power transmission line, comprising: transforming means responsive to the transmission line current in said one wire for deriving first and second alternating voltages, the peak magnitudes of said first and second alternating voltages being related to said current by first and second predetermined impedances, respectively; first means responsive to the vector sum of said first alternating voltage and a first preselected portion of the voltage across the other two wires of the transmission line for producing a unipolarity operating voltage, the peak magnitude of said operating voltage being equal to the peak magnitude of said vector sum; second means responsive to the vector difference between said second alternating voltage and a second preselected portion of the voltage across said other two wires of the transmission line for producing a relatively smooth unipolrality reference voltage, said reference voltage being related to the peak magnitude of said vector difference by a predetermined proportionality constant which is equal to the ratio of said first predetermined impedance to said second predetermined impedance, said ratio of first to second predetermined impedances also being equal to the ratio of said first preselected portion to said second preselected portion; and circuit means, including both of said unipolarity voltage producing means, for applying said operating and reference voltages in voltage opposing relationship to the electroresponsive device; whereby unidirectional energizing current will ow to the electroresponsive device in a direction determined by the relative magnitudes of said operating and reference voltages.

(References on following page) References Cited in the le of this patent UNITED STATES PATENTS Heinrich May 21, 1940 Warrington June 13, 1950 Ellis Dec. 25, 1956 Bergseth Aug. 27, 1957 Hodges July 29, 1958 12 FOREIGN PATENTS Germany Feb. 18, 1942 Great Britain Jan. 15, 1945 Switzerland Aug. 16,` 1954 Switzerland Oct. 1, 1955 Austria Dec. 10, 1957 

